Blades of gas turbines, which are usually exposed to very high hot gas temperatures, are usually produced by casting from high-strength alloys (e.g. nickel-base alloys). During the production, use is made of molds in which the pourable alloy is introduced from the lower end of the blade, from the blade root, into the mold. By virtue of a core arranged in the interior of the mold, a cooling air channel is produced in the cast blade body, which cooling air channel runs in the blade longitudinal direction through the blade body and, for cooling purposes, can conduct cooling air from the blade root to various points of the blade.
Such a blade is shown in FIG. 1: the blade 10 shown in FIG. 1 comprises an airfoil 11 which extends in the blade longitudinal direction 25 and merges into a blade root 12 at the lower end, above which blade root there is a platform 13 which inwardly delimits the hot gas passage of the gas turbine. At the upper end, the blade 10 ends in a blade tip 14, at which there is a shroud segment 15 which outwardly delimits the hot gas passage. An upwardly protruding rib 16 running in the circumferential direction of the machine can be provided on the top side of the shroud segment 15. A single cooling air channel 17, which extends in the blade longitudinal direction 25 and can be supplied with cooling air from below via a cooling air inlet 17′, is indicated by dot-dashed lines in the interior of the blade 10.
If such a gas turbine blade—as shown in FIG. 1—has an elongated design and has thin blade walls, the small cross sections between the (single) core and the mold make it difficult, during the production by casting, to introduce sufficient material from the blade root into the mold and upward into the tip, so that the relatively solid shroud segment is produced flawlessly and without cavities or porosities.
In the past, this problem has been solved either by additionally feeding material into the mold from the blade tip or by providing a second feed line on the surface of the airfoil. Such multiple feed lines are rather undesirable, however, because they can result in differently solidifying regions which impair the mechanical stability and uniformity of the mechanical properties.